1. Field of the Invention
The present invention relates to an integrated ethanol gas sensor and fabrication method thereof, and especially relates to an integrated ethanol gas sensor and fabrication method thereof, which applies micro electro-mechanical system (MEMS) technology to use a cantilever-bridge structure as a main sensor element structure, and integrates SiC heater with SnO.sub.2 thin film on the same silicon substrate via VLSI technology.
2. Description of the Related Art
There are currently three kinds of current gas sensors, based on fabrication methods: sintering gas sensors, thick film gas sensors and thin film gas sensors. The working principles of the thin film gas sensors are stated as follows.
First, a metal oxide thin film, such as SnO.sub.2, ZnO.sub.2, Fe.sub.2 O.sub.3. . . etc is deposited on a silicon substrate. When a test gas is absorbed by the metal oxide thin film, the conductivity of a sensing element is changed and the concentration of the test gas is identified.
In the case of SnO.sub.2 thin film, two main reaction modes which occur during the test, are stated as follows.
(1) Forming a Schottky contact on the thin film:
When a molecule of the test gas approaches the thin film, if the electrophilic of the test gas is higher than the power function of the semiconductor material, the test gas molecule scratches a conduction electron from the conduction band of the semiconductor material. Therefore, the conductivity of the semiconductor material is reduced. In contrast, if the electrophilic of the test gas is lower than the power function of the semiconductor material, the conductivity of the semiconductor is raised.
(2) Reaction between the test gas and the gas absorbed on the surface of the SnO.sub.2 thin film:
When the gas absorbed on the surface of the SnO.sub.2 thin film is acceptor oxygen, for example O.sub.2.sup.-1, O.sup.-1, and O.sup.-2, a depletion region is formed on the surface of the SnO.sub.2 thin film to reduce conduction electrons and increase the resistance. At this moment, the test gas, for example CO, will react with the acceptor oxygen to form CO.sub.2.sup.-1. Some electrons are excited to return to the conduction band to increase conduction electrons and reduce the resistance.
An ethanol gas sensor with a "SnO.sub.2 thin film/Ag electrode/Al.sub.2 O.sub.3 substrate" structure is stated as follows.
FIG. 1a depicts a structural diagram of a ethanol gas sensor with a "SnO.sub.2 thin film/Ag electrode/Al.sub.2 O.sub.3 substrate" structure, wherein reference numbers 11, 12, and 13 represent the Al.sub.2 O.sub.3 substrate, SnO.sub.2 thin film, and Ag electrode, respectively. FIG. 1b depicts a diagram showing a gas sensor with a Pt-coil heater 15. To improve the characteristics of the gas sensor, the operating temperature normally ranges from 300.degree. C. to 400.degree. C. Therefore, a heater is needed for heating the gas sensor to this operating temperature.
When the test gas comes in, a part of the test gas reacts with the surface of the SnO.sub.2 thin film 12 for forming a Schottky contact to enlarge the resistance the SnO.sub.2 thin film 12. This reaction belongs to mode (1). Another part of the test gas reacts with the acceptor oxygen, which is absorbed on the surface of the SnO.sub.2 thin film 12, to reduce the resistance of the SnO.sub.2 thin film 12. This reaction belongs to mode (2). The test gas is identified by a change of the resistance of the SnO.sub.2 thin film 12.
When the operating temperature is 300.degree. C., the sensitivity, reaction time, and recovery time of the ethanol gas with a concentration of 1500 ppm are 130, 10 seconds, and 30 seconds, respectively. In contrast, the sensitivity, reaction time, and recovery time of the H.sub.2 gas with a concentration of 1500 ppm are 12, 3 minutes, and 1 minute, respectively.
However, the heater of the gas sensor is not completely thermal-insulated from the outside. Thermal dissipation easily occurs at the main sensing part potentially reducing the sensitivity of the gas sensor. Further, if the material of the heater does not have good thermal stability, the heater is easily affected by the external temperature and humidity thereby making the gas sensor of the prior art unsuitable for testing gas with a high concentration. And, because the main sensing part can not be made into an array structure, the gas sensor does not perform well in a large scanning area. The total physical size of the gas sensor is not easily reduced. Therefore, the power consumption is high, and the system speed is low.